Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
2000-12-15
2003-07-08
Silverman, Stanley S. (Department: 1754)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C588S253000, C405S125000
Reexamination Certificate
active
06590133
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the field of treating materials containing lead—such as waste materials and soil—with chemical compounds which serve to reduce the bioavailability of the lead. The materials containing lead include hazardous wastes. The present invention is particularly useful in the field of treating solid wastes containing unacceptable levels of leachable lead, in order to control leaching in the natural environment and during digestion after accidental ingestion.
BACKGROUND OF THE INVENTION
Lead is one of the more common metals found in the environment, and also one of the more toxic. It is widely distributed in soil and at waste disposal sites due to its use in leaded gasoline, paint, batteries, and general widespread industrial use. The impact of lead on humans, particularly children, is being noted at increasing lower levels. Lead impacts both learning ability and behavior, particularly in children. For that reason, there is serious concern about levels of lead in the soil, and the impact of that lead on children. The Agency for Toxic Substances and Disease Registry has estimated that the number of children in the U.S. exposed to lead in soil or dust at levels of concern is between 5.9 and 11.7 million (
Impact of Lead
-
Contaminated Soil on Public Health
, ATSDR, 1992). Thus there is an increasing need to reduce the toxicity of lead in soil and waste, as well as to provide for safe disposal of lead-containing wastes.
Much of the focus of lead treatment studies until recently has been on the reduction of the leaching potential of lead from waste materials, particularly as measured in the U.S. Environmental Protection Agency's Toxicity Characteristic Leaching Procedure (TCLP) test, which is used to classify wastes as hazardous. A number of treatment technologies have been developed for immobilizing lead in wastes using phosphate, so as to render the wastes non-hazardous. See e.g. U.S. Pat. Nos. 4,737,356, 5,037,479, 5,193,936, 5,245,114, 5,430,233, 5,512,702, 5,536,899, and 5,569,155.
Less work has been done on reducing lead availability to the body following (accidental) ingestion of soil or waste. Part of the reason for this lack of emphasis on human uptake following ingestion had been the lack of an effective means of monitoring uptake during the digestion process. However, recently developed laboratory tests provide relatively simple tests for simulating the uptake during digestion, notably the Physiologically Based Extraction Test (PBET) developed by Ruby et al. See Ruby et al., “Lead Bioavailability: Dissolution Kinetics under Simulated Gastric Conditions”,
Environmental Science and Technology,
26:1242-1248 (1992); Ruby et al., “In Situ Formation of Lead Phosphates in Soils as a Method to Immobilize Lead”,
Environmental Science and Technology,
28:646-654 (1994); Ruby et al., “Estimation of Lead and Arsenic Bioavailability Using a Physiologically Based Extraction Test”,
Environmental Science and Technology,
30:422-430 (1996), and the test developed by Gasser et al., “Lead Release from Smelter and Mine Waste Impacted Materials under Simulated Gastric Conditions and Relation to Speciation”, Environmental Science and Technology, 30:761-769 (1996).
The Ruby et al (1992) Physiologically Based Extraction Test (or PBET test) involves a two step process. In the first step, the material being tested is exposed to an acidic (HCl) solution containing the digestive enzymes pepsin, citrate, malate, lactic acid, and acetic acid. The slurry is mixed for one hour at 37° C., then a small sample is taken for dissolved metal analysis. The remaining solution is neutralized to pH 7 and bile salts and pancreatin are added. The slurry is mixed for 4 more hours, then the solution is analyzed for dissolved metals. The first step models the digestive conditions found in the stomach, while the second step represents the digestive conditions as the chyme (digesting material) moves out of the stomach and into the small intestines.
Many of the treatment processes for lead in industrial wastes or soil involve the addition of orthophosphate to form insoluble lead phosphates. See the patents listed above. Phosphate, and especially phosphate combined with chloride, can reduce the solubilized concentrations of lead from a contaminated soil in the simulated digestion test leachates, as shown in Table 1.
TABLE 1
Lead Concentrations in Simulated Digestion Test Leachates: in
Untreated Soil and in Soil Treated with Phosphate and Phosphate
plus Chloride.
Simulated Digestion Test Leachate
Lead Concentrations, mg/L
Sample
Stomach
Intestines
Soil 1
Untreated
17.1
12.4
+1.1% Phosphate (as TSP)
5.25
4.74
+1.1% Phosphate and 0.3% NaCl
4.69
3.63
Soil 2
Untreated
125
52.1
+1.1% Phosphate (as TSP)
23.2
21.7
The dosages given in Table 1 above and in the Tables hereinbelow are based on the weight of additive to the weight of soil sample, both on an “as is” basis (i.e., wet weight).
In simulated digestion (PBET) tests run on contaminated soils, there is only a slight reduction in lead concentrations as the material goes from the acid to the neutral stages of the test. Since one would expect the solubility of lead to go down with the increase in pH from acid to neutral conditions, the results suggest that the lead is being complexed (and hence stabilized in solution) by the digestive enzymes. Lead concentrations in the leachate representing the intestinal phase of the simulated digestion tests are still well over the solubility levels found for the soil at neutral pH values. Further reduction in the lead concentrations requires either that the complexes between the lead and digestive enzymes be broken or that a more effective stabilization method be developed.
During digestion, the material passes first through the stomach, where it is subjected to quite acidic conditions, then into the small intestines where the pH is raised to near neutral. In both areas, digestive enzymes are introduced that aid in the breakdown of the food. The low pH of the stomach acid, combined with the complexing capacity of the digestive enzymes creates a very aggressive environment for lead solubilization from the ingested soil or waste. Most lead compounds are quite soluble in the acidic solutions found in the stomach. Once the lead is solubilized, the digestive enzymes can complex the lead to maintain it in solution as the stomach contents (or chyme) is neutralized. Further, the lead contents of most soils and many wastes is comparatively low (i.e., less than 1% by weight), which further enhances the solubilization of lead from the soil. Thus any treatment process that will immobilize lead during digestion must be particularly effective at lowering the solubility of lead, or in reducing the contact between lead and the digestive solutions.
As evidenced by Table 1, addition of phosphate or phosphate plus chloride can reduce the solubility of lead. Addition of phosphate results in the formation of the insoluble lead phosphate compounds, such as pyromorphite or chloropyromorphite. These compounds are the least soluble common lead compounds in environmental samples. The use of phosphate for immobilizing lead in regulatory leaching tests or in the environment has been documented. See Nagle et al., “Treatment of Hazardous Foundry Melting Furnace Dusts and Sludges”,
American Foundrymen's Society Transactions,
87:767-785 (1983); Ruby et al., “In Situ Formation of Lead Phosphates in Soils as a Method to Immobilize Lead”,
Environmental Science and Technology,
28:646-654 (1994); Ma et al., “In Situ Lead Immobilization by Apatite”,
Environmental Science and Technology,
27:1803-1810 (1993); and Berti et al., “In-Place Inactivation of Pb in Pb-Contaminated Soils”,
Environmental Science and Technology,
31:1359-1364 (1997). However, further reduction in lead solubility in the PBET test is needed to reduce the uptake of lead during the very acidic conditions encountered during digestion.
With no treatment, lead is very soluble in the acid pH region (pH <5). In the stomach—w
Birch & Stewart Kolasch & Birch, LLP
Kuhar Anthony
National University of Singapore
Silverman Stanley S.
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